1. Field of the Invention
The present invention relates to broadband communication systems and particularly to systems for transmission and reception of pseudorandom noise and spread spectrum signals, and to code division multiple access systems with application to satellite and terrestrial communications.
2. Description of the Related Art
Spread spectrum systems have been used for many years in digital communications. A spread spectrum signal consists of a baseband message signal modulated onto a carrier and thereafter spread in frequency by a pseudorandom noise sequence ("PN sequence" or "PNS"), independent of the message signal itself. The receiver then recovers the message signal by using a replica of the PN sequence. The main advantages of spread spectrum systems are good interference and noise rejection, low power density, ability to access multiple channels (such as in code division multiple access (CDMA) systems), high resolution ranging, and message protection. The ratio of the bandwidth of the PNS to that of the message signal, called the processing gain, determines the merit of the system.
Typical block diagrams of a spread spectrum transmitter and receiver are found in J. K. Holmes, Coherent Spread Spectrum Systems (Wiley 1982), and reproduced as FIGS. 1a and 1b. In the transmitter in FIG. 1a, a digital message signal transmitted at bit rate B is provided to coder 100. This coder encodes the data bits into codewords for transmission and can be a block coder or a convolutional coder as described in G. C. Clark and J. B. Cain, Error-Correction Coding for Digital Communications (Plenum Press 1981). Carrier frequency generator 112 generates a carrier frequency signal that is modulated by the coded signal in carrier modulator 106. PNS modulator 108 then further modulates (or spreads) the modulated carrier signal with a PN sequence from PNS generator 128. The PNS is a digital signal made up of "chips" and whose chip interval or chip period is much smaller than the data bit period (thus the bandwidth of the PN sequence is much greater than that of the data signal). The resulting signal is amplified by amplifier 130 and transmitted by antenna 140.
As depicted in FIG. 1b, the transmitted signal is received by antenna 152 and amplified by amplifier 154. Because the phase and frequency of the received signal is unknown, the received signal must be acquired and tracked to establish phase synchronization. The received signal is provided to tracking and acquisition (T&A) synchronism device 164 which contains a PNS generator that generates a replica of the PNS that was used in the transmitter. In the acquisition stage, a coarse alignment between the replicated PNS and the received signal is performed using serial and/or sequential search, sequential estimation, universal timing, or matched filter algorithms. These techniques are described in various references, one of which is R. C. Dixon, Spread Spectrum Systems with Commercial Applications (Wiley 1994). The acquisition stage brings the replicated PNS and the received signal within half a chip interval of each other. Once the received signal is acquired, the two signals are tracked, generally using a delay-lock loop. See, e.g., J. J. Spilker, Digital Communications by Satellite (Prentice Hall 1977). Once synchronized, T&A synchronism device 164 outputs the in-phase PNS to PNS demodulator 156 which demodulates (despreads) the received signal. The despread signal is provided to carrier restoration and phasing module 166 which provides a local oscillator signal which is phase-synchronized to the carrier signal. The local oscillator signal is used to demodulate the despread signal in coherent detector 158 producing a baseband coded data signal. The baseband signal is provided to clock frequency extractor 168 to extract the clock signal, which in turn is provided to decision circuit 160 which can be implemented as an integrator over a bit period to determine whether a code bit is a one or a zero. Decision circuit 160 provides squared-up digital data code bits to frame synchronization device 170 and noise-immune decoder 162. Frame synchronization device 170 uses the clock signal from clock frequency extractor 168 to extract the word (or frame) timing from the digital data code bits to derive a frame synchronization signal that is used to decode the digital data signal in noise-immune decoder 162 to recover the original digital message signal.
The main problem associated with this spread spectrum system is that the receiver contains a sequence of modules--tracking and acquisition 164, carrier restoration 166, clock extraction 168, and frame synchronization 170--each of which must wait for the previous module to acquire synchronization before being able to start its own synchronization process. The throughput of the system is therefore dependent upon each of the modules and can suffer if only one is slow to synchronize. Another problem is that the receiver requires four separate modules that duplicate some functions, i.e. phasing is performed in T&A synchronism device 164 and in carrier restoration and phasing module 166, and clock frequency extractor 168 is needed because the phasing performed by T&A synchronism device 164 is not accurate enough to clock decision circuit 160.
An improved prior art system is depicted in FIGS. 2a and 2b. In the transmitter in FIG. 2a, coder 204, carrier modulator 206, PNS modulator 208, carrier frequency generator 212, amplifier 230, and antenna 240 perform as in FIG. 1a. New to this transmitter is a second output from PNS generator 228 that provides a pulse corresponding to the beginning of each period of the PNS to switch 222 and period multiplier 210. Switch 222 then provides a clock to buffer memory 202 to synchronously clock the data bits into coder 204. The multiplication factor in period multiplier 210 is variable; thus, the period of the data bits clocked through to coder 204 can vary but is always an integral multiple of the PNS period. The use of this relationship between the data bit period and the PNS period enables the system to eliminate the clock frequency extractor from the receiver in FIG. 2b. In its place are switch 268 and period multiplier 272 which detect the clock, but do so at an earlier stage than in FIG. 1b because period multiplier 272 is directly connected to T&A synchronism device 264. The rest of the receiver works as before; thus, when the baseband coded data signal reaches decision circuit 260 from coherent detector 258, clock synchronization has already been performed and only frame synchronization remains to be performed.
One advantage of this system over the prior art system in FIGS. 1a and 1b is that the circuitry is simpler because the clock frequency extractor is more complex than the switches and multipliers in the transmitter and receiver that substitute for the clock frequency extractor. Another advantage is that the receiver operates more quickly because clock synchronization is performed simultaneously with carrier restoration.
However, there are still problems with this spread spectrum system. First, the bulk of the receiver still operates serially, reducing receiver throughput. Second, the frame synchronization device takes time to generate a frame synchronization signal and has low noise immunity due to the possibility of elementary signal distortion inside the frame synchronization signal, especially when communicating with moving objects. There may also be false start-ups in frame synchronization device 270 when there is low redundancy in the frame synchronization signal. Third, the operation of this type of system is limited when the PNS period is on the order of several chips, because the discrete nature of the spectrum radiated by the transmitter decreases the bandwidth of the system, lowering channel capacity when operating in code division mode and reducing noise immunity and interference rejection. These drawbacks may be lessened by lengthening the PNS period, which creates a more continuous spectrum, but that significantly complicates the receiving equipment, lengthens acquisition and tracking time, and worsens correlation functions in the receiver. This lowers the effective signal base which in turn reduces channel capacity and interference rejection.
The present invention addresses the shortcomings in these prior art systems. Thus, it is an object of the present invention to provide a spread spectrum system with better noise immunity, faster receiver synchronization, simpler electronics, and increased channel capacity and interference rejection.